A first method known as a method of measuring glucose concentration in blood is a method of irradiating an infrared laser beam on a part of a living body such as a finger, dispersing the scattered light from a blood vessel, and measuring glucose in the blood, as described in Patent Document 1. This method utilizes the fact that the scattered light decreases in proportion to the glucose concentration. This method, however, has a problem that the light intensity of the scattered light is dependent on temperature, moisture and oil component of the skin etc., and is therefore not popular in actuality.
A second method is a method of making the polarized component that is orthogonal to the optical rotation component transmit, and then measuring birefringence thereof in an open loop, as described in Non-patent Document 1 and Patent Document 2, etc. However, according to this method, it is impossible to measure approximately 10 mm-thick glucose with a healthy person's blood sugar level of approximately 0.1 g/dL (deciliter) in blood or a living body such as a finger because of a large measurement error.
A third method is a method of measuring using the birefringence measurement apparatus described in Patent Document 3. This method, in the same manner as the present invention, measures optical rotation of a specimen by providing an opposing collimator optical system in the ring of a ring interferometer comprised of polarization preserving fibers, making a collimated beam propagate within the specimen, and measuring the phase difference between the clockwise and the counter-clockwise propagating polarized light. This method allows measurement of approximately 10 mm-thick glucose with a healthy person's blood sugar level of 0.1 g/dL, placed in a glass cell with sufficient accuracy.
FIG. 23 is a view illustrating a conventional optical system where a light-scattering specimen is inserted between the opposing collimators. Lenses 3-1 and 3-2 are deployed at positions at focal distances 5-1 and 5-2 of the lenses 3-1 and 3-2 from ends of a pair of single mode (referred to as SM hereafter) optical fibers 1-1 and 1-2 having ferrules 2-1 and 2-2, respectively, thereby constituting an opposing collimator optical system, a light-scattering specimen 4 is placed therebetween, and optical rotation of the light-scattering specimen 4 is measured. As the best method for increasing measurement accuracy, it has a structure where ends of respective optical fibers are deployed at the focal positions of the lenses, respectively, so as to form the opposing collimators, an optical signal emitted from the end of one of the optical fibers is collimated by the lenses and irradiated onto the light-scattering specimen 4, and the optical signal transmitted through the light-scattering specimen 4 is coupled with the other optical fiber. However, when approximately 1.5 mm-thick webbing between fingers is inserted between the opposing collimators of FIG. 23, scattering loss of the living body is great, and optical rotation cannot be measured.
This is because while insertion loss of the single mode optical fiber opposing collimator optical system is normally approximately 0.5 dB, insertion loss is 80 dB or greater if the living body is inserted therewithin.
FIG. 24 shows theoretical calculation results of beam angle dependency in the case where focal distance f of the lenses of the opposing collimators using the SM optical fibers for wavelength of 850 nm is 2.5 mm and distance between the lenses is 30 mm, where in the drawing, the horizontal axis gives collimator angle (unit: degree) or angle of incoming and outgoing beams to and from the collimators, and the vertical axis gives loss (unit: dB). This shows that the coupling loss increases by 50 dB or greater if the beam angle deviates approximately zero to 0.3 degrees. Therefore, the reason that the insertion loss becomes 80 dB or greater when finger webbing is actually sandwiched by single mode optical fiber opposing collimator optical systems with lenses is considered that the light beam collimated by the lenses is scattered randomly within the living body, thereby changing the beam propagating angle.
Until now, there has been much attempt in development of optical measurement apparatuss for measuring glucose concentration in living bodies and drawn blood with high accuracy as can be seen from the above. However, the real situation is that it is very difficult to measure the glucose concentration in living bodies and drawn blood with high accuracy, and a measurement apparatus for measuring glucose concentration in living bodies and drawn blood is not developed yet, although it is used for measurement of sugar content in fruit, and thereby measurement of glucose concentration in living bodies and drawn blood must rely on methods using reagents.